The present invention relates generally to a picture data recording/reproducing system. More particularly, the invention is concerned with a picture data recording/reproducing system which is capable of recording picture data at a high compression rate and reproducing the picture data selectively at a normal reproduction speed and a higher reproduction speed.
As a system for recording and reproducing dynamic picture (image) data known heretofore, there may be mentioned a system which is disclosed in JP-A-62-164391 and in which a picture coding transmission apparatus is employed.
FIGS. 1 and 2 of the accompanying drawings are block diagrams showing a general arrangement of a moving picture recording/reproducing system based on the picture data transfer scheme known heretofore. More specifically, FIG. 1 shows a moving picture recording apparatus designed for recording on an optical disc digitized moving picture data after compression thereof, and FIG. 2 shows a moving picture reproducing apparatus for decoding the picture data read out from the optical disk and expanding the data on a real time basis for thereby displaying the moving picture. In FIG. 1, a reference numeral 11 denotes an input terminal for a picture signal, 12 denotes generally a moving picture coding unit, 13 denotes an optical disc recording unit, and a numeral 14 denotes an optical disc storage. The moving picture recording apparatus is constituted by the moving picture coding unit 12 and the optical disc recording unit 13. On the other hand, the moving picture coding unit 12 is composed of an analogue-to-digital (A/D) converter 15, a frame memory 17, a picture coding circuit 18 and a buffer memory 20.
Referring to FIG. 2, a numeral 21 denotes an optical disk reading unit, 22 generally denotes a moving picture decoding unit, and 23 denotes an output terminal for the reproduced picture signal. The moving picture reproducing apparatus is constituted by the optical disc reproducing unit 21 and the moving picture decoding unit 22, which in turn is composed of a buffer memory 25, a picture decoding circuit 26, a frame memory 28 and a digital-to-analogue (D/A) converter 29, wherein a numeral 27 designates picture data.
In operation, the input picture signal applied to the input terminal 11 is supplied to the input of the moving picture coding unit 12, wherein the analogue picture signal is converted into digital picture data by the A/D converter circuit 15, which data is outputted onto a line 16 and thence supplied to the frame memory 17 to be stored therein. The picture coding circuit 18 encodes the picture data outputted from the frame memory 17 with high efficiency to thereby generate coded data which is then stored in the buffer memory 20 over a line 19. The coded data stored in the buffer memory 20 is then supplied to the optical disc recording unit 13, whereby the coded moving picture data transferred from the moving picture coding unit 12 is recorded on the optical disc 14. Of course, in the case of a read-only optical disk in which data can not be written by the user, such as a CD-ROM (Compact Disc--Read Only Memory) and others, a prototype is first prepared, from which replica discs are manufactured by press equipment.
Upon reproduction of the optical disc 14 having the coded moving picture data recorded thereon by the optical disc reading unit 21, the coded data read out from the optical disc 14 is supplied to the moving picture data decoding unit 22. More specifically, the coded picture data outputted on the line 24 is first stored in the buffer memory 25 and subsequently decoded by the picture data decoding circuit 26, whereby the decoded picture data is generated. The decoded picture data thus generated is supplied to the frame memory 28 over a line 27 to be stored and held therein. Subsequently, the decoded picture data is read out from the frame memory 28 and undergoes D/A conversion in the D/A converter circuit 29, the resultant analogue picture signal being then outputted as the output picture signal through the terminal 23.
With regard to the moving picture data coding scheme, there are known an intra-frame coding scheme and an inter-frame coding scheme, as is disclosed, for example, in JP-A-61-24577. According to the intra-frame coding scheme, the moving picture or picture data is coded completely in each of the data frames, as typified by a DPCM (Differential Pulse Code Modulation) according to which the difference between the current data value and the before or after data value is determined to be coded. On the other hand, according to the inter-frame coding scheme, the moving picture data is coded on a frame basis (i.e. frame by frame) by making use of frame information in the past, as exemplified by a changed-area coding scheme according to which the coding is performed only for the part of the data that has undergone a change from that of the preceding frame data. In general, the inter-frame coding scheme can assure a higher ratio of data compression than the intra-frame coding system, presenting the advantage that the amount of the coded data is correspondingly reduced.
FIG. 3 of the accompanying drawings shows in a block diagram an exemplary structure of the coding circuit 18 implemented on the basis of the DPCM principle, and FIG. 4 shows in a block diagram a structure of the decoding circuit 26 which is compatible with the DPCM coding circuit 18. Referring to FIG. 3 , a reference numeral 30 denotes a data value subtraction circuit, 31 denotes a quantitizing circuit for quantitizing data with a decreased number of the quantitizing levels to thereby reduce further the amount of data to be processed, 32 denotes a representative value setting circuit, 35 denotes a data value addition circuit, and a numeral 34 denotes a data value hold circuit. The data value substraction circuit 30 serves for determining the difference between the value of the picture data 16 inputted currently thereto and the immediately preceding value of the picture data held in the data value hold circuit 34. The difference data thus derived is then quantitized by the quantitizing circuit 31 with the number of quantitizing levels being decreased to an appropriate value, whereby coded data is generated to be outputted onto the line 19. In the representative value setting circuit 32, the coded data undergoes a reverse quantitization for restoring the original bit number by adding "Os" in a number corresponding to the number of bits deleted by the quantitizing circuit 31 to be subsequently added to the value of the immediately preceding picture data by the data value addition circuit 33. In this manner, the picture data is, so to say, locally decoded through cooperation of the circuits 32 and 33. The picture data resulting from the local decoding mentioned above is stored and held in the data value hold circuit 34 to be thereby delayed by one picture element. Now, referring to FIG. 4, a numeral 35 denotes a representative value setting circuit, 36 denotes a data value addition circuit and 37 denotes a data value hold circuit. The coded data 24 undergoes a reverse quantitization in the representative value setting circuit 35 and is subsequently added by the value addition circuit 36 to the value of the immediately preceding picture data stored and held in the data value hold circuit 37 to be thereby decoded. The picture data generated on the line 27 as a result of the decoding is stored and held in the data value hold circuit 37 to be delayed by one picture element.
FIG. 5 is a schematic diagram for illustrating conceptually a changed-area coding scheme exemplifying typically the inter-frame coding scheme. In a frame generally designated by 38 in this figure, pictures depicted in a solid line belong to the frame being generated currently, while pictures depicted in broken lines are those of the immediately preceding frame. According to the changed-area coding system, only the regions 39 each indicated as enclosed by a thick solid line need to be coded, which means that a high ratio of compression can be accomplished. For coding the picture data within the changed areas or regions, there can be employed, for example, the DPCM intra-frame coding system. In this case, it is however noted that the frame memories for holding the picture data of the immediately preceding frame must be provided in association with the coding circuit 18 and the decoding circuit 26, respectively, because the data of the immediately preceding frame has to be made use of upon coding and decoding, respectively.
Next, referring to FIGS. 6A to 6C, discussion will be made concerning a recording format for the coded moving picture data and a reproducing method adopted in the prior art dynamic picture reproducing/recording apparatus described hereinbefore in conjunction with FIGS. 1 and 2. FIG. 6A illustrates disposition or array of the coded moving picture data recorded on a track of the optical disk 14, and FIGS. 6B and 6C are views for illustrating a method of reproducing the coded data. As will be seen in FIG. 6A, the coded picture data is recorded on a frame basis in the order of the frame numbers assigned sequentially to a series of frames 58. The data compression is so realized that the amount of coded data for one frame becomes smaller than the maximum amount of the data which can be transferred within a period taken for displaying one frame (1/30 sec.) and which is determined on the basis of the maximum data transfer rate in the data reproduction from the optical disc. Upon reproduction of the moving picture data at an ordinary or normal speed, the coded data is read out on a frame basis in the order of the frame numbers, as shown in FIG. 6B. On the other hand, upon reproduction of the moving picture data at a high speed, e.g. at a trebled speed, the coded data is read out every third frame, as shown in FIG. 6C. It should further be added that in case the inter-frame coding scheme is adopted, the high speed reproduction based on the frame thinning-out or skipping mentioned above can not be realized.
When the intra-frame coding scheme is adopted, the high speed reproduction is certainly possible, because the picture data can be coded completely and independently within each frame. However, a difficulty will be encountered in an attempt to perform a recording for an extended time because of the impossibility of compressing the picture data at a relatively high ratio. On the other hand, when the inter-frame coding scheme is adopted, the picture data can certainly be compressed at a high compression ratio when compared with the case where the intra-frame coding is adopted, because the data compression is realized by making use of correlation of the picture data between the frames in the case of the inter-frame coding. However, because of the impossibility of coding the data independently within each frame according to the inter-frame coding, an attempt for high-speed reproduction of the picture data by reading out only the necessary frames is rendered impractical.